PURPOSE: To reduce the dependency of a branching ratio upon polarization and to use a photocoupler film of six-layered structure even at a large angle of incidence by reducing the 1st layer dielectric film which has a high refractive index, the 2nd layer dielectric film which has a low refractive index, and the 6th layer dielectric film in film thickness to λ/4 and increasing the 4th layer dielectric film which has a low refractive index and the 5th layer dielectric film in film thickness.
CONSTITUTION: The 1stWthe 6th layer dielectric films 1W6 are formed on a glass substrate 10; the thicknesses t1, t2, and t3 of the 1st, the 3rd, and the 5th layer dielectric films 1, 3, and 5 have high refractive indexes and the 2nd, the 4th, and the 6th layer dielectric films 2, 4, and 6 have low refractive indexes. The 1st, the 2nd, and the 6th layer dielectric films 1, 2, and 6 are decreased to 1/4 of wavelength λ, and the film thicknesses t4 and t5 of the 4th and the 5th layer dielectric films 4 and 5 are increased. The polarization dependency of the branching ratio decreases even to a large angle of incidence as the difference in transmissivity between a P wave and an S wave decreases. Therefore, stable optical branching and mixing are performed.
KUSAKA SATOSHI
NODA HIDEKI
